1. Field of the Invention
This invention relates generally to electric motors, and more particularly to a method of making a rotor for electric motors in which rotor magnets and a rotor frame are integrated by a resin.
2. Description of the Prior Art
The prior art has provided outer rotor type electric motors in which a bottomed short cylindrical rotor having a relatively large diameter is used. The rotor comprises a bottomed short cylindrical frame having a large diameter and made of a steel plate and a number of rotor magnets mounted on an inner circumferential surface of an annular wall of the frame. With regard to mounting the rotor magnets on the inner circumferential surface of the frame wall, the prior art has proposed a method of integrating the rotor magnets and frame by means of a resin. More specifically, the rotor magnets and frame are accommodated in a cavity of a forming mold. Subsequently, a molten resin is poured into the cavity to be hardened so that the rotor magnets and frame are enclosed in the hardened resin to be integrated together.
The thickness of the annular wall of the frame needs to be increased in the above-described rotor so that magnetic paths are sufficiently secured for the rotor magnets. However, the increase in the thickness of the annular wall results in an increase in the thickness of a bottom wall of the frame. That is, the bottomed short cylindrical frame is formed by pressing a steel plate and accordingly, the thickness of the steel plate needs to be increased so that the thickness of the annular wall of the frame is increased. This also increases the thickness of the bottom wall of the frame. Thus, the entire thickness of the frame is increased. This disadvantageously increases the entire weight of the rotor.
Therefore, an object of the present invention is to provide a method of making a rotor for the electric motor wherein the magnetic paths can sufficiently be secured for the rotor magnets without an increase in the entire thickness of the frame and with an increase in the weight of the rotor being restrained, the method having a high working efficiency.
The invention provides a method of making a rotor for an electric motor including a stator, the method comprising steps of accommodating, in a forming mold, a plurality of annularly disposed rotor magnets, a frame made of steel plate and formed generally into a shape of a bottomed short cylinder, the frame having an annular wall disposed outside the annularly disposed rotor magnets so as to be located at a side opposed to the stator with respect to the rotor magnets, the annular wall having an open end, and a ring member made of a magnetic material and disposed along the annular wall of the frame at an inner or outer circumferential side of the annular wall, the rotor magnets being held by magnet holding means provided in the forming mold in execution of the accommodating step, and pouring a molten resin into the forming mold accommodating the rotor magnets, the frame and the ring member and hardening the resin.
According to the above-described method, the work for accommodating the rotor magnets, the frame and ring member into the forming mold can be readily performed.
In the above-described method, the axial ends of the rotor magnets projecting outward from the open end of the annular wall of the frame are fitted into concave portions formed in the forming mold so that the rotor magnets are positioned. Consequently, the rotor magnets can readily and accurately be positioned relative o the forming mold.